U.S. patent number 4,166,783 [Application Number 05/897,235] was granted by the patent office on 1979-09-04 for deposition rate regulation by computer control of sputtering systems.
This patent grant is currently assigned to Varian Associates, Inc.. Invention is credited to Frederick T. Turner.
United States Patent |
4,166,783 |
Turner |
September 4, 1979 |
Deposition rate regulation by computer control of sputtering
systems
Abstract
A sputtering system utilizes a computer to monitor the power
dissipation in the sputtering source and to accumulate the history
of usage of the particular sputtering target. Desired deposition
rate information is input to the computer, which establishes and
maintains the desired rate and controls the plasma discharge to
compensate for aging and deterioration of the target. End of useful
target life is determined by the computer from objective criteria
to trigger appropriate actions.
Inventors: |
Turner; Frederick T.
(Sunnyvale, CA) |
Assignee: |
Varian Associates, Inc. (Palo
Alto, CA)
|
Family
ID: |
25407589 |
Appl.
No.: |
05/897,235 |
Filed: |
April 17, 1978 |
Current U.S.
Class: |
204/192.13;
204/298.03 |
Current CPC
Class: |
C23C
14/3407 (20130101); H01J 37/3479 (20130101); H01J
37/3444 (20130101); H01J 37/34 (20130101) |
Current International
Class: |
C23C
14/34 (20060101); C23C 015/00 () |
Field of
Search: |
;204/192R,298 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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|
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2422808 |
|
Nov 1975 |
|
DE |
|
2232832 |
|
Feb 1975 |
|
FR |
|
7237840 |
|
Sep 1972 |
|
JP |
|
Primary Examiner: Mack; John H.
Attorney, Agent or Firm: Cole; Stanley Z. Berkowitz; Edward
H.
Claims
What is claimed is:
1. In a sputtering system comprising
a cathode and an anode spaced from said cathode,
controllable excitation means for supporting a plasma discharge
between said cathode and said anode,
means for monitoring the power dissipated in said discharge,
means for monitoring the duration of operation for said cathode,
and
computation means for continuously correcting said excitation means
in response to a function of said duration and said power for
maintaining the deposition rate of material at a selected
value.
2. The apparatus of claim 1 comprising hermetic envelope means for
containing a desired gas in the region of said discharge.
3. The apparatus of claim 2 wherein said function has dependence
upon the pressure of said gas and said system further comprises
means for monitoring said pressure.
4. The apparatus of claim 3 wherein said computation means includes
means for correcting said excitation means in response to said
pressure dependence whereby said deposition is maintained at said
selected value.
5. In a sputtering apparatus having a cathode and an anode in a
hermetically sealed enclosure and means for maintaining a discharge
between cathode and anode, the method of maintaining the sputtering
deposition rate of such apparatus at a related value comprising the
steps of,
introducing a selected gas into the enclosure to a desired
pressure,
maintaining a plasma discharge between anode and cathode,
logging the cumulative usage time during which said cathode is
subject to discharge,
sensing the parameters of said discharge, said parameters
comprising the current drawn from said cathode and the potential
difference between said cathode and anode,
computing the power dissipated in said discharge,
correcting a parameter of said discharge in accord with a function
relating deposition rate, cumulative usage time, and said power,
whereby said deposition rate is compensated for deterioration of
the cathode.
6. The method of claim 5 wherein said function exhibits dependence
upon the pressure in said enclosure and the parameters of said
discharge further include said pressure.
7. The method of claim 6 wherein said step of correcting includes
said pressure dependence.
Description
BACKGROUND OF THE INVENTION
The present invention involves an improved sputtering system,
computer controlled to compensate for both long term and short term
deviations from desired performance.
In the deposition of thin films, control of parameters such as film
thickness, uniformity and the like are of critical importance. For
most coating applications, and particularly in a production
context, sputtering techniques have proven to be advantageous in
achieving desired specifications for a wide variety of materials.
It is known, however, that certain components of sputter coating
apparatus are subject to deterioration; for example, the target
(cathode), which is the source of sputterant, erodes under ion
bombardment in such fashion as to significantly alter the geometry
of the cathode surfaces during the useful life thereof. U.S. patent
application No. 805,485, now U.S. Pat. No. 4,100,055, assigned to
the assignee of the present invention, discloses an improved
initial cathode shape which yields a longer useful life and thus
greater integrated mass transfer of sputterant than achieved in the
prior art. It has been pointed out in the above-referenced
application that the power dissipated in the plasma, as well as the
cathode geometry, is subject to a dependence on cumulative usage of
the particular cathode. It is apparent that the rate of deposition
reflects a similar variation. In the prior art, an independent
deposition rate monitor provided, a deposition rate signal for use
in a feedback loop to control the excitation source of the plasma
discharge.
A BRIEF DESCRIPTION OF THE PRESENT INVENTION
The present invention employs a digital computer to stabilize the
sputtering rate at a desired level which is selected by operator
input to the computer. The computer accomplishes this by regulating
the operation of a closed loop system wherein the power dissipated
by the excitation source is monitored from the current drawn from
the cathode and the cathode-anode voltage. Moreover, the empirical
relationship of deposition rate to the cumulative history of the
type of cathode is retained by the computer and accessed by the
computer to correct the power dissipated in the sputtering
discharge in order to maintain the desired deposition rate from the
presently installed cathode. The computer also monitors the
cumulative history of the presently installed cathode in order to
update this correction periodically. The desired deposition rate is
obtained from information input to the computer independently and
determines a reference level of deposition rate to which operation
is stabilized. The computer determines from objective criteria
based upon experience when the end of useful life for the
particular cathode is reached and takes appropriate action in that
event.
Accordingly, it is an object of the present invention to stabilize
sputtering deposition rate at a desired value.
In one feature of the present invention the prior history of the
specific cathode is accumulated and retained by a computer.
In another feature of the invention the power dissipated in the
sputtering plasma discharge is monitored by said computer.
In yet another feature of the invention the operating parameters of
the excitation source for said sputtering apparatus is controlled
by said computer in response to the monitored power dissipated, the
cumulative history of the sputtering cathode the pressure within
the sputtering chamber and the desired rate of deposition.
In yet another feature of the invention the aging characteristics
of the particular type of cathode are retained and accessed by said
computer to correct the operation of the apparatus for the aging of
the cathode then in use.
Other features and advantages of the present invention will become
apparent upon a perusal of the following specification, taken in
connection with the accompanying drawings wherein:
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 depicts the aging characteristics of one type of sputtering
target.
FIG. 2 is a schematic illustration of the system of the present
invention.
FIG. 3 is a flowchart of the computer routine for the present
system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The sputtering process is initiated by the impact of positive ions,
typically argon, on a cathode surface. The impact dislodges cathode
material which deposits on the object to be coated. The source of
positive ions is a plasma excited by the potential maintained
between cathode and anode. In a well-known form a cathode structure
surrounds a centrally disposed anode. The sputtering process
inevitably erodes the cathode surface and the electrical
characteristics are altered thereby. Consequently, the deposition
rate for given operating conditions deteriorates. FIG. 1 is an
empirical measurement of the performance of a cathode of given
material and geometry such as the Varian model C 655859 in a Varian
S-gun (TM) as a function of its cumulative operation expressed in
kilowatt hours at constant pressure. The ordinate is the deposition
rate per kilowatt normalized to the same parameter at an initial
stage of usage. The data of FIG. 1 refer to constant pressure which
is ordinarily independently monitored and regulated. The present
invention is not limited to S-Gun systems and could be employed
with magnetron sputtering sources as well.
A preferred embodiment of the present invention is schematically
indicated in FIG. 2. A sputtering apparatus 2 comprises an anode 5
and cathode structure 7 with auxiliary structures 9. The latter
include shield electrode and magnets for containing the plasma and
directing the impacting ions. More detailed discussion of
sputtering systems for coating applications may be found in
Hoffman, Solid State Technology, Dec. 1976, pp. 57-66. The anode
potential with respect to ground is maintained by a bias supply 10
and the cathode-anode voltage is monitored by voltage sensor 12.
The anode potential vs. ground is typically maintained at a value
of +40 volts. Voltage sensor 12 is preferably a digital volt meter
which may be interrogated by digital computer 14.
The cathode current is supplied by variable current power supply 16
which maintains the cathode at a low potential with respect to
ground. Typically the cathode is maintained in a range of about
400-1000 volts negative with respect to ground and may draw
currents ranging from 3 A to 10 A in accord with the desired
deposition rate and the age of the particular cathode among other
parameters. The cathode current is sensed by current sensor 18
which is also monitored by computer 14. The current drawn from
cathode supply 16 is controlled by computer 14 in response to the
power dissipated in the plasma, the cumulative usage of the
particular target, the pressure and the desired deposition
rate.
The computer 14 is first employed to perform certain initialization
computations. For example, in a production context an operator
specifies the material and thickness of coating desired and
throughput of object to be coated. The computer determines the
deposition rate and the initially required power in view of the
elapsed usage of the particular cathode and controls the system
accordingly. Thereafter, the operation of the digital computer is
outlined conveniently by the flowchart of FIG. 3. The cathode anode
potential and the current in the cathode-anode loop is obtained
from sensors 12 and 18 and the power is calculated. The nature of
the calculation performed is as follows. The deposition rate, power
dissipation and the aging characteristic are expressed by an
empirically obtained function specific to the cathode material
which is stored in the computer;
where P is the power, r is the deposition rate, .tau. is the
integrated "age" of the cathode in kilowatt hours and .rho. is the
pressure. For a desired rate R specified by the operator, the above
equation may be solved to obtain the required power. This may be
obtained by calculation using appropriate numerical techniques or
by a table look-up. It will be apparent that this routine is
executed outside the loop for initialization and inside the loop to
correct the power for the usage of the cathode. The duration of
cathode usage is incremented and the kilowatt hours of use updated
accordingly, preferably by reference to the internal real time
clock of computer 14. When the usage of the cathode exceeds a
predetermined amount the computer branches to a stop condition. In
normal course the elapsed kilowatt hours of usage is used to access
an aging function stored in the computer, which function is
peculiar to the type of cathode employed. A correction factor is
then obtained for altering the current control of cathode power
supply 16. Program control then returns to the top of the loop.
Pressure sensing apparatus 20, monitors the pressure in the
neighborhood of the plasma discharge. The pressure dependence of
equation 1 may be explicitly compensated or, alternatively, the
pressure may be maintained constant at a desired value by an
independent servo-loop. The latter is most commonly accomplished by
regulating the plasma gas supply in relationship to the pumping
speed of the vacuum pumps of the system.
It is appreciated that the stop condition may include initiating
operation of another sputtering source or altering production
parameters such as the time for which the workpiece is to be
subject to coating. Other measures consistent with a stop condition
will readily occur to one skilled in the production coating
arts.
The present apparatus does not employ a deposition rate sensor to
complete the feedback loop. The sputtering source itself provides
sufficient information to the computer to permit regulation and
correction. Thus, in a system comprising a plurality of sputtering
sources regulated by a single computer, it is therefore not
required to provide another plurality of deposition rate monitors
for local monitoring of the sputtering sources. Because of the
complexities of deposition rate monitors, great economics are
thereby achieved in eliminating such monitoring apparatus and
reducing the data handling requirements of the computer.
From the foregoing it will be seen that the present invention
provides a useful method and apparatus for stabilizing deposition
rate of sputterant from a sputtering apparatus without the direct
monitoring of deposition rate by a deposition rate sensor. It will
be understood that many modifications of the structure of the
preferred embodiment will occur to those skilled in the art, and it
is understood that this invention is to be limited only by the
scope of our claims.
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